The present invention relates to a back light device for uniformly illuminating a liquid crystal display plate, or more in particular to a fluorescent lamp back light device using a fluorescent lamp, in which the luminance of the liquid crystal display plate is improved without any unevenness even for an increased size of the liquid crystal display plate.
In the case where a straight-tube fluorescent lamp is used as a back light source in the back of a liquid crystal display plate, the luminance, which is high on the surface of the liquid display plate nearer to the tube part of the fluorescent lamp, progressively deteriorates toward the parts of the liquid crystal display screen away from the tube, thereby causing what is called a luminance unevenness on the surface of the liquid crystal display plate. In order to obviate this luminance unevenness, as disclosed in JP-A-No. 60-75910(U), a shade plate for removing the luminance unevenness is interposed between the fluorescent lamp and the liquid crystal display plate, so that the light emitted from the fluorescent lamp is transmitted through the shade plate thereby to reduce the luminance unevenness on the surface of the liquid crystal display plate.
In the prior art described above, the shade plate for removing the luminance unevenness interposed between the fluorescent lamp and the liquid crystal display plate causes the reflection and absorption of light thereon, thereby reducing the luminance on the surface of the liquid crystal display plate as a whole. Also, the provision of the shade plate undesirably increases the number of parts required.
Further, if the luminance is to be improved by providing a plurality of straight-tube fluorescent lamps, a proportionately increased number of light circuits are required which is accompanied by an undesirable increased cost of the device.